1. Field of the Invention
The present invention relates to extruded thermoplastic sheeting consisting of a pair of sheets or layers spaced apart and interconnected by extending ribs so that the interior of the boards contains a plurality of extending passageways. More particularly, it relates to thermoplastic sheeting consisting of a pair of sheets or layers, which are substantially parallel to each other and are interconnected by extending ribs. The hollow thermoplastic sheeting of the present invention significantly enhances the surface flatness of thermoplastic sheeting as compared to prior art products. The present invention also relates to the process and composition for production thereof.
2. Information Disclosure Statement
Hollow thermoplastic panels, which are made of thermoplastic resin, are widely used in the sign, lamination and graphic art applications. A number of processes for production are already known to those skilled in the art. The following references illustrate the state of the art in this field:
One method in the prior art, as described in U.S. Pat. Nos. 3,509,005, 3,664,906, 3,748,214 and 3,741,857, for the manufacture of such lightweight board integrally molds a sheet with a plurality of ribs extending from the surface of the sheet. Another sheet of plain structure or having a plurality of extending ribs from the surface of the sheet, can be bonded to the previous sheet by bringing the two sheets together under heat-softened conditions such that the two sheets heat bond to one another.
U.S. Pat. Nos. 5,910,226 and 3,837,973 use a method for the manufacture of hollow thermoplastic boards, which consists of two or three extruders. The material from the middle extruder is molded into shapes by a roller and is united with the films from the other two extruders into one member by fusing together while they are under heat-softened conditions.
A pressure is applied when the sheets are united together by fusion to establish connection at their mutually contacting parts, in the previous techniques. The hollow thermoplastic sheeting produced according to the previous techniques has a plurality of ridges arising from the flat sheet along the contacting lines of the flat sheets and ribs, which significantly affects the flatness of the surfaces.
U.S. Pat. Nos. 3,274,315, 3,792,951, 4,513,048 and 5,658,644 use a process, which integrally extrude the two sheets and the plurality of the ribs of the hollow thermoplastic board through an extrusion orifice having a corresponding orifice configuration. The extruded boards then enter a calibrator, which cools and shapes the dimension of the board. The boards manufactured by such method consisting of a pair of sheets or layers spaced apart and interconnected by longitudinally extending ribs so that the interior of the boards contains a plurality of extending straight passageways.
The plastic hollow lightweight boards manufactured by the above method do not show a plurality of ridges at the areas where the flat sheets and the extending ribs are joined. In contrast, the contacting areas sink to form a plurality of depression bands extending along the passageway direction, which negatively affect surface flatness. The depression bands are especially apparent for polymers of high crystallinity such as polypropylene, high-density polyethylene, etc. It is believed that the depression bands are due to the thermal contraction and crystallization of the polymeric material in the extending ribs.
The plurality of depression bands or ridges on the surfaces of the hollow thermoplastic sheeting are unfavorable in the applications such as printing and laminating substrates. When being used as a printing substrate, the plurality of ridges or depression bands of the hollow thermoplastic sheet surface on the printed graphic art, incur undesired reflection of light and negatively affect the printing resolution and quality. In addition, more printing ink per unit area and greater press pressure may be necessary to cover the areas around the depression bands or ridges, which increase ink consumption and may shorten the life spans of the printing plates or screens. While being used as a laminating substrate, the topology of the hollow thermoplastic sheeting may appear on the surfaces of the laminated films or sheets.
The hollow thermoplastic boards can be laminated with films or sheets of smooth surface to improve the surface flatness. However, the extra materials, including the films or sheets and the adhesives to glue the sheets or films to the hollow thermoplastic boards, and the additional laminating process substantially increase the cost of the hollow thermoplastic sheeting of smooth surfaces.
It is therefore the object of the present invention to provide a hollow thermoplastic board, which has improved surface smoothness and does not incur significant increase of production cost.
It is also the object of the present invention to provide methods and compositions to economically produce hollow thermoplastic boards of substantially improved surface smoothness.
The smooth hollow thermoplastic board consisting of a pair of sheets or layers, which are substantially parallel to each other and are interconnected by extending ribs, and the corresponding process for the production thereof disclosed in this invention are believed to be neither taught nor render obvious by the prior art.
An extruded hollow thermoplastic board is disclosed which has a pair of flat and parallel sheets spaced apart and interconnected by extending ribs. The extending ribs are foamed to expand and extend the rib sections of the hollow thermoplastic boards. The expansion of the rib section compensates the shrinkage of the rib sections due to thermal contraction and the crystallization of the thermoplastic material when it cools after exiting the extrusion die. Consequently, the depths of the depression bands on the surfaces of the hollow thermoplastic boards ax reduced. The surface smoothness is substantially enhanced, which is highly desirable for applications such as printing, lamination and graphic art. The present invention also provides methods for the manufacture of the hollow thermoplastic boards of smooth surfaces.